Shells for wind turbine blades forming the aerodynamic profile of the blades, i.e. the airfoils, are commonly manufactured by laying up of fibre reinforcements in respective upper and lower mould halves. The upper and lower mould halves are used to form two half shells—a first half shell provides the suction surface of the blade and a second half shell provides the pressure surface of the blade. The two half shells are joined together along a leading edge and a trailing edge to form the blade.
To fabricate each shell half, fibre reinforcement material (such as glass fibre and/or carbon fibre) is laid up in each mould half. Next, a vacuum film is placed over the fibre material. The vacuum film is commonly referred to as ‘vacuum bagging film’ and is sealed against the half mould to eliminate air leaks and create a substantially sealed volume containing the fibre reinforcement material. Air is then removed from the substantially sealed volume using a vacuum pump. The vacuum pump extracts air from the substantially sealed volume and from the lay-up of fibre reinforcement material to create an effective vacuum, which causes the vacuum film to apply pressure to the fibre reinforcement material. Under the vacuum, resin (typically thermoset resin) is infused into the fibre material and the mould half is heated to cure the resin. When both half shells have been manufactured in their respective half moulds, the two half moulds are brought together to close the mould and to adhesively join the half shells along the leading edge and the trailing edge to form the blade.
Instead of the resin being infused into the substantially sealed volume, the fibre reinforcements may be pre-impregnated with a thermoset resin (i.e. pre-preg fibre material), which is heated to above its glass transition temperature under the vacuum, to cause the resin to distribute evenly within the mould and bond the fibre reinforcements together.
The trailing edge section of wind turbine blades poses a particular challenge in relation to manufacturing by moulding. From an aerodynamic perspective, it may be desired to minimize the thickness of the trailing edge of airfoils for wind turbine blades in order to minimize aerodynamic drag. For obvious reasons, however, indefinitely thin trailing edges cannot be achieved, and accordingly wind turbine blades generally have a certain trailing edge thickness of between a few millimeters and a few centimeters. So-called flat back profiles with a considerable trailing edge thickness have been proposed in the prior art. It will hence be appreciated that the trailing edge effectively has a non-zero thickness in traditional blades and flat-back structures alike. Upper and lower mould halves used for the manufacture of such blades are normally split along a pressure side, i.e. lower surface of the blade. Accordingly, the upper mould half defining and accommodating the suction side of the blade also defines the flat-back trailing edge section and accommodates the fibre reinforcement, which forms the trailing edge section, the fibre reinforcement being generally provided as a mat. At the trailing edge of the blade, the blade forms a 90° or almost 90° corner, in which fibre mats may not sit effectively unless the corner is rounded. In order to avoid a fragile resin rich area in such corners, a transition between the pressure side surface of the blade and the trailing edge may hence be rounded, i.e. provided with a corner radius. Such a radius may, however, compromise aerodynamic performance of the blade, because it reduces the aerodynamically effective area of the airfoil and impedes controlled flow separation.